Chopper stabilized operational amplifiers (op amps) reduce DC input offset voltages. These characteristics make chopper op amps ideal for precision, low frequency applications such as strain gauge and thermocouple amplifiers. However, problems associated with noise and aliasing arise when current chopper op amps are used in higher frequency applications.
Op amps commercially available today use internal chopping frequencies in the range of 100 Hz to 400 Hz to ensure low DC input offset voltages on the order of 5 microvolts or less. The use of a chopping frequency tends to limit the application of the op amp to input signal frequencies up to one-half of the chopping frequency. At input frequencies higher than one-half of the chopping frequency, aliasing problems occur. Aliasing is the result of large noise spikes which occur at harmonics of the summation of, and difference between, the chopping frequency and the input frequency. For example, for an op amp which chops at a frequency of 500 Hz, an input frequency of 200 Hz, will give rise to an aliasing spike at 300 Hz and 700 Hz, and harmonics thereof.
Commercially available chopper op amps also often actually increase noise over linear op amps when used in applications above 10 Hz. Thus, while noise is reduced with the use of chopper op amp below 10 Hz, increases in frequencies beyond the 10 Hz spectrum bring an increase in noise level associated with the amplified signal.
Therefore, a need has arisen for a chopper op amp which may be utilized in larger frequency bandwidth applications while ameliorating aliasing problems and reducing noise associated with the chopper op amp output.